We learned this, and everything else in this post from a single source briefed on Microsoft's mobile strategy.

Though several other sources in the mobile industry described this news as plausible, logical, and even likely, none were able to fully corroborate it. So take what you read here with a grain of salt.

Briefed on the details of this story, a Microsoft PR rep declined to comment on "rumors."

Facebook PR also declined to comment on this story.

Until now, reports about the Facebook Phone have almost universally suggested that, after originally planning to write its own mobile operating system from scratch, Facebook has since decided to make its mobile operating system a "forked" version of Google's open source mobile OS, Android. Amazon did something similar with the Kindle Fire.

But according to our source, Microsoft does not believe Facebook has gone so far down that road yet that it can't come back and do something else.

Microsoft would like a Facebook Phone built on a version of the Windows Phone to be that "something else."

Microsoft Windows Phones already have deep Facebook integration. Users can sign into their phones with their Facebook IDs and sync their calendars and address books. This integration would go deeper. The phone would be branded differently too. Imagine something like: The Facebook Phone, powered by Windows.

We know what to do and how to do it. Making a phone is not easy. You have to develop the OS, get manufacturers to build handsets for it, get carriers to carry it, market it, sell it, and ship it. Facebook has no idea how to do any of this. Microsoft does, and it will offer to help Facebook with all of it.

We know everyone. Microsoft has relationships across the industry. Samsung, HTC, and Nokia are already making Windows Phones. LG was. AT&T, T-Mobile, Verizon are already carrying the phones. Microsoft spends hundreds of millions of dollars with ad agencies every year. It's working with a logistics firm, Brightpoint, to get the product shipped to stores efficiently.

Why Microsoft want to do this? Again, according to our source:

Some of the appeal is being able to drive mobile traffic to Microsoft's online properties, particularly Bing.

Microsoft's grandest desire is deep penetration in mobile that will allow it to someday offer consumers a NFC-powered payments product.

Microsoft wants to do something like PayPal, but in the physical world. It wants to get there before Google figures out how to fix Google Wallet.

Microsoft is not particularly interested in the concept of location-based advertising.

What's the timing? Not imminent. Our source says Microsoft is "hanging around the hoop," hoping to be there when Facebook finally gets serious about phone development.